"We arrested fourth accused Abhishek, 25, from Mandawli in east Delhi Tuesday night," said a police officer.
On Feb 26, Abhishek along with Amit (maternal uncle of the victims), Pankaj and Shivam killed Mansij, 7, and his sister Yashvi, 5, after abducting them from school, the officer said.
The three accused were arrested March 3, police said.
They demanded Rs 30 lakh from the victim's father Manoj Kumar for their safe release, police said.
Three days after the abduction, the victims' bodies were spotted by a taxi driver in the bushes near the Ring Road-Bhairon Road crossing in central Delhi.